1. Field of the Invention
The present invention relates to a positive electrode active material having a layered rock-salt structure for lithium secondary batteries.
2. Description of the Related Art
Positive electrode active materials using a lithium complex oxide (lithium transition metal oxide) having a layered rock-salt structure are widely known as positive electrode active materials in lithium secondary batteries (which may be referred to as lithium ion secondary batteries) (see, for example, Patent Document 1 (JP5-226004A) and Patent Document 2 (JP2003-132887A)).
It is known that, in positive electrode active materials of this type, diffusion of lithium ions (Li+) therein takes place along an in-plane direction of the (003) plane (i.e., any direction in the plane parallel to the (003) plane) while intercalation and deintercalation of lithium ions occur through a crystal plane other than the (003) plane (e.g., the (101) plane or the (104) plane).
Thus, for positive electrode active materials of this type, attempts have been made to enhance battery performance of lithium secondary batteries by exposing a crystal plane through which lithium ions are favorably intercalated and deintercalated (i.e., a plane other than the (003) plane, such as the (101) plane or the (104) plane) to a surface which comes into contact with a larger amount of electrolyte (see, for example, Patent Document 3 (WO2010/074304)).
In addition, in positive electrode active materials of this type, those having pores (also referred to as holes or voids) formed therein are known (see, for example, Patent Document 4 (JP2002-75365A), Patent Document 5 (JP2004-083388A), and Patent Document 6 (JP2009-117241A)). Furthermore, a positive electrode active material having voids inside, in which primary particles are oriented in the one axial direction in secondary particles, is also known, and this configuration makes it possible to obtain high output performance and rate performance (see, for example, Patent Document 7 (WO2012/137391).